1. Field of the Invention
The present invention relates to an operating voltage point compensation circuit for a transistoried circuit. In particular, the present invention pertains to an operating voltage point compensation circuit that resolves a problem arising from the saturation of a direct current operating voltage point , which is the result of a power supply voltage change or a temperature change, in a low voltage drive circuit that is constituted by a transistorized circuit.
2. Description of the Related Art
Recently, various types of transistorized circuits driven at low voltages have been adopted to save power consumption. But when a transisterized circuit is driven at a low voltage of, for example, +3.3 V, a direct current operating voltage point may become saturated.
In FIG. 5 is shown an example of cascade connection by which differential transistors are coupled with emitter followers. Transistors TR1 and TR2, which are coupled differential transistors for which collector resistors R1 and R2 are respectively provided, have their emitters connected in common to a constant-current source and their collectors connected to the bases of the respective transistors TR3 and TR4, which serve as emitter follower circuits.
The emitters of the transistors TR3 and TR4 are connected to the bases of coupled differential transistors TR5 and TR6, and the emitters of the differential transistors TR5 and TR6 are connected in common to a transistor TR7.
Let us first consider a case where Vcc=+3.3 V. When Vcc=V1+V2+V3+3 Vbe, and when the base-emitter voltage Vbe of the transistor is 0.8 V (at junction point temperature Tj=25.degree. C.), V1+V2+V3=0.9 V.
Therefore, in the case where, due to a voltage change, Vcc=+2.97 V (a 10% reduction), and due to a temperature change, Vbe=0.87 V (at junction point temperature Tj=-45.degree. C.), "V1+V2+V3" yields a total of 0.36 V; and if V2=0 V, the total yield for "V1+V3" is only 0.36 V. Since the internal amplitude limit is approximately 0.2 V, an output may become saturated because of a variance in the circuit and an external change.